Major advances have occurred in the last ten years in techniques for observing behavior under natural conditions. However, limited attention has been given to traditional psychometric issues of validity and reliability. The purpose of this analysis was to provide a guide to literature on reliability in observational research. Current problems in definition and theory, practical applications, and sequential analysis were explored. BIBLIOGRAPHIC REFERENCE: Hollenbeck, A.R. Problems of reliability in observational research. In G.P. Sackett (Ed.), Observing Behavior, Volume 2: Data Collection and Analysis Methods, Chapter 5. University Park Press: Baltimore, in press 1977.